The objectives of this project are: 1. To determine the nature of the antigenic determinants on the hemagglutinin (HA) and neuraminidase (NA) molecules of influenza viruses and the molecular mechanism of antigenic drift. Our main approach will be to sequence HA molecules (and the corresponding RNA segments) of influenza virus variants occurring naturally or selected under antibody pressure. In order to monitor drift in single antigenic determinants, we will use variants selected with monoclonal antibodies. This should enable us to find out: (a) What are the exact changes which occur in the amino acid sequence of the HA polypeptides of different variants selected with the same clone of antibody and of variants selected with different clones. (b) Whether the changes in amino acid sequence occur within the antigenic sites or whether they occur somewhere else in the HA molecule and alter the determinants through conformational changes. Similar experiments will be done with NA, using naturally occurring antigenic variants. We will try to isolate antigenic fragments of the HA molecule. We will also make derivatives of amino acid side chains exposed on the surface of the HA and see if these interfere in antibody binding, and make derivatives of the HA to which antibody molecules are bound, to see if antibody "protects" any amino acid side chains. 2. To investigate the structure of those virus-coded proteins too small in quantity or too difficult to purify to sequence directly (e.g., the P and NS proteins) by sequence analysis of the corresponding RNA segments. These proteins are concerned with the growth-capacities of the virus, its host-range and its "virulence" and comparative studies of their amino acid sequences are of great interest. The main approach we will use to sequence the RNA segments of influenza virus will be to transcribe them into their complementary DNA copies and sequence them using the rapid DNA-sequencing techniques recently developed.